That doesnt always work. Alot of the 550's x2's are x2's because 1 or 2 of their cores are defective.

This is the new batch. They're ALL fully working Phenom x4s that have just been rebranded to fill the x2 market.

Not strictly true I bought a 555 x2 black edition and when I unlocked the extra cores it overheated and froze. They have all 4 cores but, to reiterate what Avonius said, they sometimes have deffective core 3 and 4. Amd use a complex system that grades the properties of chips depending on things such as thermal properties and where they were on the silicon wafer